1. Field of the Invention
The present invention relates to a master cylinder that supplies brake fluid to a brake device or the like in an automobile.
2. Description of the Related Art
As described in the microfilm of Japanese Utility Model Application No. 55-152602 (Japanese Utility Model Application Laid-Open Publication No. 57-73248), there is a conventional master cylinder (fast fill master cylinder) with which it is possible to compensate for an ineffective fluid amount at an initial stage of a stroke by so-called fast filling, in which a large amount of brake fluid is supplied at an initial stage of actuation when the brake fluid is supplied to a brake device such as a disk brake or a drum brake, and, as a result, shorten the pedal stroke.
The fast fill master cylinder is disposed with: a stepped cylinder that includes a large diameter cylinder portion and a small diameter cylinder portion; a stepped piston that includes a large diameter piston portion, which is slidably inserted into the large diameter cylinder portion of the stepped cylinder, and a small diameter piston portion, which is slidably inserted into the small diameter cylinder portion of the stepped cylinder; and a cup seal (open/close check portion) that partitions the inside of the stepped cylinder into a large diameter pressurizing chamber on the large diameter piston portion side and a small diameter hydraulic chamber on the small diameter piston portion side and permits flow of brake fluid only from the large diameter pressurizing chamber side to the small diameter hydraulic chamber side.
When the stepped piston slides toward the small diameter hydraulic chamber side by input from a brake pedal, the volume of the large diameter pressurizing chamber is reduced by the sliding of the stepped piston toward the small diameter hydraulic chamber side, whereby the open/close check portion is opened and the fluid is supplied from the large diameter pressurizing chamber side to the small diameter hydraulic chamber side. Thus, fast filling is conducted.
The fast fill master cylinder is disposed with a relief valve that allows brake fluid to escape from the large diameter pressurizing chamber to a reservoir when the pressure inside the large diameter pressurizing chamber becomes equal to or higher than a predetermined value, and a notch is disposed at the relief valve so that the large diameter pressurizing chamber always communicates with the reservoir. The fluid is supplied from the reservoir to the large diameter pressurizing chamber through a communication path that has an extremely small diameter due to the notch.
In recent years, a hydraulic control unit has been disposed on a communication flow path that connects a small diameter hydraulic chamber of the master cylinder and the brake device, in order to control brake fluid pressure of the master cylinder corresponding to the depression of the brake pedal and to transmit the brake fluid pressure to the brake device. An example of such a hydraulic control unit is an ABS unit that determines whether or not the wheels have a tendency to slip during braking and, when the wheels do have a tendency to slip, reduces pressure to a hydraulic pressure that avoids the tendency to slip and transmits the brake fluid pressure to the brake device.
A restrictor is disposed inside the hydraulic control unit. When a restrictor is disposed inside the hydraulic control unit, there are no effects of the restrictor at the time of ordinary braking when the speed of pressurization is low. However, because hydraulic pressures of a communication flow path, which connects the hydraulic control unit and the small diameter hydraulic chamber, and a branched flow path, which branches from the communication flow path, suddenly rise due to the restrictor and reach a set valve-opening pressure before the brake fluid is sufficiently supplied to the brake device, even if an open/close check portion is opened and fluid is about to be supplied from the large diameter pressurizing chamber to the small diameter hydraulic chamber side and to the brake device side in order to compensate for the ineffective fluid amount during the initial stage of the stroke at the time of sudden braking, when the speed of pressurization is high, the hydraulic pressure of the large diameter pressurizing chamber is released to the reservoir side before the brake fluid is sufficiently supplied to the brake device side. Thus, there has been the problem that it has been impossible to compensate for the ineffective fluid amount during the initial stage of the stroke. As a result, the pedal stroke extends, and pedal feel is worsened.